The role of bacterial 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity in the interaction between tomato (Lycopersicon esculentum=Solanum lycopersicum) and Pseudomonas brassicacearum was studied in different strains. The phytopathogenic strain 520-1 possesses ACC deaminase activity, an important trait of plant growth-promoting rhizobacteria (PGPR) that stimulates root growth. The ACC-utilizing PGPR strain Am3 increased in vitro root elongation and root biomass of soil-grown tomato cv. Ailsa Craig at low bacterial concentrations (10(6) cells ml-1 in vitro and 10(6) cells g-1 soil) but had negative effects on in vitro root elongation at higher bacterial concentrations. A mutant strain of Am3 (designated T8-1) that was engineered to be ACC deaminase deficient failed to promote tomato root growth in vitro and in soil. Although strains T8-1 and 520-1 inhibited root growth in vitro at higher bacterial concentrations (>10(6) cells ml-1), they did not cause disease symptoms in vitro after seed inoculation, or in soil supplemented with bacteria. All the P. brassicacearum strains studied caused pith necrosis when stems or fruits were inoculated with a bacterial suspension, as did the causal organism of this disease (P. corrugata 176), but the non-pathogenic strain Pseudomonas sp. Dp2 did not. Strains Am3 and T8-1 were marked with antibiotic resistance and fluorescence to show that bacteria introduced to the nutrient solution or on seeds in vitro, or in soil were capable of colonizing the root surface, but were not detected inside root tissues. Both strains showed similar colonization ability either on root surfaces or in wounded stems. The results suggest that bacterial ACC deaminase of P. brassicacearum Am3 can promote growth in tomato by masking the phytopathogenic properties of this bacterium.

We have determined the nucleotide sequence of aroG, the gene coding for 3-deoxy-D-arabinoheptulosonate-7-phosphate synthetase(phe), one of three isoenzymes that catalyse the first step of the biosynthesis of aromatic amino acids and vitamins in Escherichia coli K12. The DNA sequence agrees with previously published data on the N-terminal sequence, amino acid composition, and subunit molecular weight of 3-deoxy-D-arabinoheptulosonate-7-phosphate synthetase(pne). There is significant identity in the nucleotide sequences of aroG and aroH (the gene for 3-deoxy-D-arabinoheptulosonate-7-phosphate synthetase (trp), indicating that these two genes have evolved from a common ancestral gene. There is no attenuator structure in the leader region of aroG.

The promoter, operator, and 5' and 3' ends of the mRNA of the Escherichia coli gene aroG (encoding the phenylalanine-sensitive 3-deoxy-arabinoheptulosonate-7-phosphate synthase) were located. Primer extension analysis and nuclease Si mapping of in vivo transcripts were used to determine the 5' and 3' ends, respectively, of the mRNA. Both ends exhibited some heterogeneity with respect to length. The 3' end of the major molecular species was located within a region that has structural homology with known rho-independent terminators. The location of the aroG promoter was identified in both strands of the DNA by in vitro DNase I footprinting and methylation protection experiments with RNA polymerase. In these experiments, a region of up to 80 base pairs (bp) was protected by the binding of RNA polymerase. The location of the aroG operator was also identified in both strands of the DNA by in vitro DNase I footprinting with pure TyrR. TyrR protected 26 to 28 bp of DNA containing a 22-bp palindrome (TYR R box) and overlapping the'-35 region of the promoter. Mutations in the aroG regulatory DNA were isolated by site-directed mutagenesis and cloned in a low-copy-number plasmid to generate aroG-lac fusions. The effects of the mutations on the regulation of aroG expression were determined by measuring the (-galactosidase activities of the fusions in strains with tyrR, tyrR+, and multicopy tyrR' genotypes. The results of this mutant analysis confirmed that the aroG operator contains a single TYR R box.In Escherichia coli, the phenylalanine-sensitive 3-deoxy-D-arabinoheptulosonate-7-phosphate synthase (EC 4.1.2.15) is one of three isoenzymes that catalyze the first committed step in the biosynthesis of tyrosine, phenylalanine, tryptophan, and other aromatic compounds (22,36). This enzyme is encoded by the gene aroG. The expression of aroG and a number of other genes that encode enzymes and proteins required for the biosynthesis of these compounds and the uptake of the three aromatic amino acids is regulated by TyrR, the protein product of the gene tyrR (15). The eight operons that TyrR is known to regulate constitute the TyrR regulon (36). The transcription of aroG is repressed by TyrR (26) in the presence of excess phenylalanine and tryptophan (6, 7). Other operons in the TyrR regulon are repressed by TyrR in the presence of tyrosine alone or combinations of tyrosine and tryptophan or phenylalanine; the degree of maximal repression varies widely within the regulon (for a review, see reference 36). The molecular basis of the interaction of TyrR with its target operons is of interest, since the situation in which one regulatory protein exerts differential control over a number of different target genes appears to be common in both procaryotes and eucaryotes.In previous studies, we determined the nucleotide sequence of aroG and surrounding DNA (17) and used a promoter-cloning vector to localize the aroG promoter and operator within a 276-base-pair' (bp) Sau3AI fragment (18). The investigations that we describe in this communic...

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